1. Field of the Invention
The present invention relates generally to blast furnaces and more particularly to a specifically configured iron ore pellet to be used as the raw material in the blast furnace, a method of making such pellets, and a method of operating the blast furnace using such pellets.
2. Description of the Prior Art
In recent years, the ore beneficiation technique in the iron ore industry has made remarkable progress, so that powdered ores produced in mines, which had usually been discarded, are now positively utilized as the raw material for iron to be charged into blast furnaces as lump ores. Thus their value as goods has been improved to a remarkable extent. As is publicly known, the sintering method and the pelletizing method are the two principal methods for such ore beneficiation, as reflected by the high percentage of approximately 80%, which is the ratio of ores beneficiated by the two methods relative to the total amount of raw materials charged into blast furnaces in Japan. Of these beneficiated ores, the amount of production of sintered ores is over-whelmingly high, but recently that of pellets is increasing with respect to both import and domestic production, that is, it accounts for nearly 20% of beneficiated ores. In addition, there has already been established a mass production system for the so-called self-fluxed pellet which has been pre-adjusted into slag component for a more efficient operation of the blast furnace. Conventional plants can with a daily output of 8,000 tons, and thus, in some plants, a blast furnace operation with an increased blend ratio of pellets is greatly facilitated.
However, the operation of a blast furnace charged with a considerable amount of pellets does not always render favorable results as compared with the case in which a considerable amount of sintered ores are blended. Pellets possess some properties which are more advantageous than sintered ore in some aspects, but when viewed as a whole, it also has such drawbacks as are not fully satisfactory.
In particular, the drawbacks associated with conventional pellets are attributable to the physical property that they are in the form of a sphere, and this deleteriously affects the operation of the blast furnace.
Detailed reasons for the above are set out below while reference is made to FIGS. 1 and 2. When pellets are used in a blast furnace operation, pre-weighed spherical pellets of diameters ranging from 5 to 20 mm, and coke as a reducing agent, are charged in an alternate manner into a blast furnace (1) through its charging portion (2) as illustrated in FIG. 1, so that inside the furnace, a pellet layer (PL) and a coke layer (CL) are piled one upon another, that is, in a layer-by-layer manner. As a result, layers piled at the upper portion within the furnace each generally form a depression at the central portion thereof and a raised portion at the periphery, thus giving rise to a V type distribution. In this case, it is desired that the pellet layers (PL) and coke layers (CL) be uniformly piled with little change in layer thickness in the radial direction within the furnace. Actually, however, this is not realized because the coke and the pellets are markedly different in their physical properties. That is, as shown in FIG. 2, when the pellets (p) are charged into a coke layer (CL) in the furnace, there will be a larger amount of pellets flowing from the periphery toward the central portion as compared with the case of coke (C), resulting in the fact that the pellet layer (PL) formed within the furnace will have a remarkably larger layer thickness (t.sub.1) at its central portion than the layer thickness (t.sub.2) at its peripheral portion, thus causing non-uniformity in the radial direction. Furthermore, when coke is charged onto such pellet layer (PL), there will be a smaller amount of coke flowing toward the central portion than that of the pellets because the angle of repose as measured from the longitudinal axis of the furnace is larger for the coke layer than that of the pellets with the result that the coke layer (CL) formed within the furnace will have a remarkably smaller layer thickness (t.sub.1)' at its central portion than the layer thickness (t.sub.2)' at its peripheral portion contrary to the case of the pellet charging, thus also causing a non-uniform layer thickness distribution in the radial direction. When this is repeated, the inside of the furnace as a whole will be in such condition as that shown in FIG. 2, that is, there occurs a non-uniform distribution such that the pellets are accumulated in the central portion and the coke is accumulated at the periphery, so that the flowing velocity of the gas from below becomes higher at the peripheral portion and lower at the central portion as schematically indicated by the upward arrows in FIG. 2. Consequently, the temperature of the peripheral portion in the furnace becomes higher than that of the central portion, with the amount of reducing gas produced being larger at the periphery and the reduction reaction of the raw iron material being increased at the peripheral portion.
The amount of charged material flowing toward the central part of the furnace depends greatly upon the so-called angle of repose of that charged material. Table 1 shows the angle of the repose of materials charged and the angle of inclination within the furnace. As shown, the angle of repose of the pellets has small values as compared with that of coke, and this difference causes the foregoing non-uniform phenomenon to occur in the furnace. On the other hand, the values of sintered ore are substantially in the same range as those of coke, so that in the case of sintered ore, the foregoing phenomenon does not normally occur and a uniform distribution of layer thickness is obtained relatively easily. The reason why pellets cause such non-uniform phenomenon to occur is that pellets are spherical and are almost perfectly round, and have a smooth surface and therefore its contact frictional resistance is extremely lower when compared with that of sintered ore and coke that structures of which are quite non-uniform or uneven.
Table 1 ______________________________________ Material Angle of inclination charged Angle of repose in furnace ______________________________________ Pellet 25- 28 20- 26 Sintered ore 31- 34 29- 31 Coke 30-35 33- 38 ______________________________________
As set forth hereinbefore, the flowing of conventional pellets toward the central part of the furnace, and the resulting non-uniform distribution of the layer thickness causes the coke layers to become disordered and the flow of reducing gas to be biased toward the peripheral portion or to become non-uniform and unstable, and there also occurs a disordered furnace condition such as an unbalanced descent in the furnace of the materials being charged, resulting in the reduction reaction in the furnace being impeded and the operational efficiency thereof being lowered. Furthermore, even after being piled in the furnace, the pellets will normally undergo vibration or irregular movement due to the flow of gas therethrough, and consequently will be incorporated into the adjacent coke layer, thus causing the thickness of the coke layer to become non-uniform and the permeability of the furnace to be varied and the reactivity of coke deleteriously affected. To be more specific, it becomes impossible to increase the ore/coke ratio which results in decreased productivity and increased coke consumption.
It is known that the reduction reaction of the pellets proceeds topochemically from the periphery towards the center, but within a high temperature portion, there is formed at the periphery a close and hard metallic iron layer which is the reduction product, which impedes the invasion of the reducing gas to the pellet interior so that an unreacted nucleus is liable to remain at the interior of the pellet. Such a drawback is also attributable to the spherical shape of the pellet. The residual unreacted portion causes a lowering in the softening and melting points of the pellet and also may cause a phenomenon of fusion between pellets. Due to the spherical shape of the pellets, moreover, the condition in the furnace approaches that of close charging, in which condition there are only a small number of spaces within the pellet layer, whereby such phenomenon of fusion is further promoted. It goes without saying that such fusion in the pellet layer results in poor permeability of the reducing gas and an efficient operation of the blast furnace is rendered difficult.